Because of the height reached by wind turbines and their location on elevated areas, they have a high risk of receiving lightning strikes particularly at its blades. Therefore the blades must have a lightning protection system and any additional system installed in them containing conductive elements (metal parts, sensors, beacon systems, etc.) must be protected against direct lightning strikes and indirect effects due to the electromagnetic field induced by them.
The primary components of lightning protection system for wind turbine blades are a series of metal receptors mounted on the surface of the blade and a down conductor to drive the lightning from the receptors to the blade root. The evolution in wind turbines development together with the growth of the power provided by them has led to new generations of wind turbines having increasing dimensions both in tower height and rotor diameter. Blade lengthening involves an increase in rigidity. The use of more carbon fiber laminates in the manufacturing of blades is a conventional way to achieve this rigidity whether as “caps” joined to the shells or forming part of the shell structure. However, carbon fiber laminates are conductors and must therefore be connected in parallel with the down conductor of the lightning protection system to prevent internal arcing between the down conductor and the laminates as well as direct lightning strikes on the carbon laminates.
WO 2006/051147 A1 describes a lightning protection system comprising means for equipotentializing the carbon fiber laminates with the lightning protection system including derivations from the down conductor to connect it directly to the carbon fiber laminates arranged in the upper and lower parts of the beam of the blade (see FIG. 1a). An auxiliary cable 5 derived from down-conductor 6 is connected by a bolted joint to a metal plate 3 in direct contact with the carbon fiber layers of the beam flanges 4 that serves as a means for injecting current into them. The electrical connection can be improved by using conductive resins shaped as carbon nanofibers or nanotubes 11 added both between the metal plate 3 and the carbon fiber layers as in the vicinity of the metal plate 3. Because the high frequency of the lightning current, the local current distribution in the connection, causes that the effective connection area is less than the physical, being the current density much greater in area A near the auxiliary cable 5 that in area B far away from it.
The present invention is directed to solving that problem.